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Ambient Temperature Correction Factor Task Group

Ambient Temperature Correction Factor Task Group. Ambient Temperature Correction Factor Task Group. Maintainer Installer Larry Ayer, IEC, Chairman Stan Folz – NECA Arizona Carmon Colvin, IEC, Alabama Labor Jim Dollard, IBEW, Co-Chair IAEI Donny Cook, IAEI – Alabama

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Ambient Temperature Correction Factor Task Group

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  1. Ambient Temperature Correction Factor Task Group

  2. Ambient Temperature Correction Factor Task Group • Maintainer Installer • Larry Ayer, IEC, Chairman • Stan Folz – NECA Arizona • Carmon Colvin, IEC, Alabama • Labor • Jim Dollard, IBEW, Co-Chair • IAEI • Donny Cook, IAEI – Alabama • Patrick Richardson, IAEI Tamarack Florida • Manufacturers • Alan Manche, NEMA • Research and Testing • Bill Fiske, Intertek • Dave Dini, UL • Tim Shedd, Professor Univ of Wisc Madison • William Black, Professor Georgia Tech

  3. William Z. Black received his BS and MS in Mechanical Engineering from the University of Illinois in 1963 and 1964, respectively, and his PhD in Mechanical Engineering from Purdue University in 1967. Since taking his doctorate, he has been at the George W. Woodruff School of mechanical Engineering at the Georgia Institute of Technology, where he is presently Regent's Professor and the Georgia Power Distinguished Professor of mechanical Engineering. He has directed a number of EPRI projects relating to ampacity of underground cables and overhead conductors. He is on several IEEE ampacity committees and is a member of CIGRE Committee 22.12 on the thermal behavior of overhead lines. He is a registered Professional Engineer in Georgia. William Black, Phd Member, IEEE/ICC Committee 3-1 Ampacity Tables Member, IEEE/ICC Committee 12-44 Soil Thermal Stability Member, IEEE Standard  442-1981 WG Member, IEEE Standard on Soil Thermal Resistivity Working Group Member, ICC/IEEE Standard 835-1994 Working Group Member, IEEE Standard. 738-1993 Working Group Member, IEEE/ICC Transient Ampacity Task Force Member, Emergency Ratings of Overhead Equipment Task Force Member, IEEE Thermal Aspects of Bare Conductors and Accessories Working Group Member, IEEE/ICC, Working Group C24, Temperature Monitoring of Cable Systems Chairman, IEEE/ICC C34D Committee on Mitigating Manhole Explosions

  4. Direct applications of this work are spray cooling of high heat flux electronics, boiling and condensation in smooth and enhanced tubes, and the development of cleaner, more efficient small engines through a better understanding of carburetor behavior. We are approaching this through the use of unique experimental flow loops and flow visualization techniques. Long, clear test sections are used to study a range of fluids and flow conditions. New optical measurement techniques, such as Thin Film PIV, are being developed to quantify flow behavior. Results from these measurements will be fed into efforts to develop accurate, flexible and computationally efficient models for use both by university researchers and system designers in industry. Though he has several areas of interest, Tim's current focus is on identifying the primary mechanisms responsible for two-phase heat and momentum transfer in thin films. While this may sound a little esoteric, these conditions exist in literally millions of appliances and commercial products world wide. A better understanding of the behavior of vapor-liquid systems can lead to improved efficiencies, less waste materials (refrigerants and heat exchangers), and greater affordability of products. Tim Shedd, Phd

  5. Task Group Approach Reviewed Historical Information Conference Call – invited all concerned parties to express their views. Discussed if any known failures if they had occurred. Reviewed UL/CDA and IAEI papers Developed Heat Transfer Model with UW-Madison Developed Public input for CMP-6

  6. 1889-Kennelly Historical • 1894 Insurance Co. set at 50% • 1896 Insurance Co. revised to 60% • 50C Code Grade Rubber

  7. 1940-Present Rosch Used basic Heat Transfer Equation to determine ampacity Ampacity for Conductors in free air Ampacity for Conductors in conduit 1938 Rosch Used basic Heat Transfer Equation to determine ampacity Ampacity for Conductors in free air Ampacity for Conductors in conduit

  8. 1938-1940 I Current Flow 0V 120V Resistance of copper conductor Q Heat Flow Q 50 30C Thermal Resistance

  9. Heat Transfer of Cable

  10. Heat Transfer within Conduit 90 30 R2 Air Resistance Inside Conduit R4 Conduit to Air Resistance R3 Conduit Resistance R1 Insulation Resistance

  11. Heat Transfer Conduction through Insulation Natural Convection outside conduit Radiation in Radiation out Forced convection outside (wind) Forced convection inside (wind, chimney effect) Natural Convection inside conduit

  12. 1984-1987 • Proposals to NEC • Neher-McGrath Method 1956 • Corrected Rosch – 1938 • Considered to be more accurate • Included in 1984 NEC for adoption in 1987 • Most parts rejected in 1987 due to termination concerns • Retained for medium voltage • Moved to Annex B for low voltage

  13. Proposal 6-41 (1984) • The NEC is very conservative in its ratings of bare and covered conductors (line wire). • The NEC does not employ a technique to account for the effect of sun and wind. • The NEC does not correctly account for the difference in ampacity of bare and covered line wire. • The NEC ratings for not more than three conductors in a raceway can cause both the inspector and the user to make significant errors because: • They do not provide for the variables of load factor and earth thermal resistivity in underground applications. • There is no derating factor that will get one to the most common earth ambient - 20°C. • For most direct burial applications the NEC will waste money because it is too conservative. • For conduit-in-air applications, the NEC ratings are too conservative.

  14. Proposal 6-41 1984 COFFEY (UL Representative) : I am voting againstthe Panel recommendation to accept this proposal even though I agree it is technically correct.My negative vote is based on: (i) its far-reaching impact on equipment and installations covered by many other parts of the Codeand, (2) the need for coordination with those parts of the Code that are effected by changes in the ampacity rating of conductors. I recommend that a study be made to assess the overall impact of these changes and to identify any needed modifications to other provisions of the Code.

  15. Numerical Model of Wire Heating Timothy A. Shedd 29 September 2014

  16. Univ of Wisc-Madison Report When conduit is in contact with roof surface the conductor temperature is highly dependent on the roof surface temp. When the roof surface is 77 deg C, the conductor temp rise above ambient is approximately 33C above ambient. When roof surface is 42C, conductor temperature rise above ambient is 7.2C. When conduit is raised off the roof, conductor temperature is approximately 22.8C above the ambient. Numbers obtained from model are in-line with numbers from UL fact-finding report.

  17. Roof Wiring systems mounted directly on roof Add 33C Celsius Roof Wiring systems raised off roof Add 22C Celsius

  18. Roof Convection Reflected Solar Radiation Solar Radiation Rooftop Conduction Roof Convection Reflected Solar Radiation Solar Radiation

  19. Case 4: 3 No. 12 AWG in ¾” EMT ¾” EMT raceway O.D. 0.92 in =23.4 mm ID = 0.824 in = 21 mm Wall = 0.049 in = 1.25 mm Galvanized steel k_s = 51 W/m-K emissivity = 0.83 absorptivity = 0.7

  20. Assumptions in model • Tamb = 41 °C (105.5 °F) • No forced air movement external to conduit (only natural convection) • No axial air movement internal to conduit • Absorption coefficient α = 0.7 (from NREL database) • Emission coefficient ε = 0.83 (from NREL database, where ε = 0.88; adjusted downward to match UL study data; Pessimistic adjustment) • Natural convection coefficient = 6 W/m2K • Resistance between wire and conduit = 0.5 K-m/W (from finite element simulation) • Solar radiation 1050 W/m2(UL results only use data for insolation between 1000 and 1100 W/m2) • I = 0 A (for comparison with UL data) • Temperature-variable model of wire resistivity used • Radiation only through upper half of conduit (both absorption and emission; net radiative exchange with roof assumed negligible)

  21. Results – Compare to UL measurements Twire,mod = 63.3 °C; ΔTamb = 22.5 °C (40.4 °F)

  22. Results – I2R losses included • I = 20 A (per wire) • Twire,mod = 75.6 °C; ΔTamb = 34.7 °C (62.5 °F) • I = 25 A (per wire) • Twire,mod = 82.7 °C; ΔTamb = 41.9 °C (75.4 °F)

  23. Case 15: 3 500 kcmil in 4” EMT 4” EMT raceway O.D. 4.5 in =114.3 mm ID = 4.334 in = 110.1 mm Wall = 0.083 in = 2.11 mm Galvanized steel k_s = 51 W/m-K emissivity = 0.83 absorptivity = 0.7

  24. Results – Compare to UL measurements Twire,mod = 61.6 °C; ΔTamb = 20.7 °C (37.3 °F) emissivity increased to 0.88 (NREL value)

  25. Results – I2R losses included • I = 430 A (per wire) • Twire,mod = 80.6 °C; ΔTamb = 39.7 °C (71.5 °F) • I = 380 A (per wire) • Twire,mod = 76.2 °C; ΔTamb = 35.4 °C (63.7 °F)

  26. UL / CDA Report infers rooftop issue is linear Example • 41 degree C ambient in Nevada • 33 degree C ambient Temp Rise in Conduit due to Radiation • 50 degree C rise due to fully loaded conductor. 124 degree C rise Total

  27. UNLV Report • All conduits tested were raised off roof 8 inches. Did not compare with conduits on roof to test for affects of roof conduction. • Circuit had 13.3 amps. Well short of NEC allowable limits.

  28. UNLV Report Each of the wiring methods experienced a temperature rise that exceeded the ambient temperature. In the case of the energized conductors, which were the minimum allowable size for the continuous load carried, the maximum temperature experienced was 69° C, approximately 77% the temperature rating of the conductor insulation (i.e., 90° C). In the case of the non-energized conductors, the maximum temperature experienced was 60° C, approximately 67% the rated temperature of the conductor insulation. Since this is an experimental setup and not a working installation, the measured temperatures are likely higher than a real-world installation due to the complete exposure of the entire conduit length including origination points. Real-world installations usually terminate on a rooftop, but originate in lower ambient temperature locations such as in an electrical room or on the side of a building.

  29. Findings Heat Transfer is complex. CDA / UL Report do not take into account electrical loading in conduit CDA / UL Report do not take into account how conduits are terminated. CDA / UL Report assume that Heat Transfer outdoors is linear when it is not. If conduits are not elevated above roof conductor temperature can be elevated above 90C due to added conductive heat transfer from roof. 1000 W/m2 solar radiation. 1000 W/m2 is based maximum solar radiation during a one or two hours a day, during one or two months out of a year. When considering full loading of conductors, conductors inside conduits raised off roof will be below the 90C threshold.

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